jest spyon function without object

Jest spyOn a function not Class or Object type

stackoverflow.com › questions › 49457451 › jest-spyon-a-function-not-class-or-object-type

I am not expert in jest, but my recommendation to think about:

1) When the function is exported as default I use something like:

import Funct1 from "../../../src/components/Funct1";
...
jest.mock("../../../src/components/Funct1");
...
expect(Funct1).toHaveBeenCalledWith(params);

2) When the module (utils.js) has multiple exports as

export const f1 = () => {};
...
export const f8 = () => {};

You can try

import * as Utils from "../../../src/components/utils"
const f8Spy = jest.spyOn(Utils, 'f8');
...
expect(f8Spy).toHaveBeenCalledWith(params);

Similar discussion here

Answer from Dima Dorogonov on Stack Overflow

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stackoverflow.com › questions › 49457451 › jest-spyon-a-function-not-class-or-object-type

Jest spyOn a function not Class or Object type

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r/reactjs on Reddit: How to mock only a non-default function in Jest

November 6, 2023 -

I am trying to mock a non default function in jest which is part of a larger module and I don't want to mock any of the rest of it. I tried something along the lines of the solution found here (https://stackoverflow.com/questions/59312671/mock-only-one-function-from-module-but-leave-rest-with-original-functionality). The problem is that importing the default function gives you the whole module instead of the default function.

For example:

// myModule.js
export const foo = () => {
    console.log("in foo");
};

const bar = () => {
    console.log("in bar");
};

export default bar;

// useMyModule.test.js
import bar, { foo } from "./myModule";
import "./useMyModule";

jest.mock("./myModule", () => ({
    ...jest.requireActual("./myModule"),
    foo: jest.fn(() => {
        console.log("in test foo");
    }),
}));

test("see if this works", () => {
    console.log(bar);
    foo();
});

// useMyModule.js
import bar from "./myModule";

console.log(bar);
//bar(); // <== commented out because bar is not the function I expect it to be

This is what the output looks like:

 PASS  src/useMyModule.test.js
  ● Console

    console.log
      {
        default: [Function: bar],
        foo: [Function: mockConstructor] {
          _isMockFunction: true,
          getMockImplementation: [Function (anonymous)],
          mock: [Getter/Setter],
          mockClear: [Function (anonymous)],
          mockReset: [Function (anonymous)],
          mockRestore: [Function (anonymous)],
          mockReturnValueOnce: [Function (anonymous)],
          mockResolvedValueOnce: [Function (anonymous)],
          mockRejectedValueOnce: [Function (anonymous)],
          mockReturnValue: [Function (anonymous)],
          mockResolvedValue: [Function (anonymous)],
          mockRejectedValue: [Function (anonymous)],
          mockImplementationOnce: [Function (anonymous)],
          mockImplementation: [Function (anonymous)],
          mockReturnThis: [Function (anonymous)],
          mockName: [Function (anonymous)],
          getMockName: [Function (anonymous)]
        }
      }

      at Object.<anonymous> (src/useMyModule.js:3:9)

    console.log
      {
        default: [Function: bar],
        foo: [Function: mockConstructor] {
          _isMockFunction: true,
          getMockImplementation: [Function (anonymous)],
          mock: [Getter/Setter],
          mockClear: [Function (anonymous)],
          mockReset: [Function (anonymous)],
          mockRestore: [Function (anonymous)],
          mockReturnValueOnce: [Function (anonymous)],
          mockResolvedValueOnce: [Function (anonymous)],
          mockRejectedValueOnce: [Function (anonymous)],
          mockReturnValue: [Function (anonymous)],
          mockResolvedValue: [Function (anonymous)],
          mockRejectedValue: [Function (anonymous)],
          mockImplementationOnce: [Function (anonymous)],
          mockImplementation: [Function (anonymous)],
          mockReturnThis: [Function (anonymous)],
          mockName: [Function (anonymous)],
          getMockName: [Function (anonymous)]
        }
      }

      at Object.<anonymous> (src/useMyModule.test.js:12:13)

However, if you uncomment the bar() invocation above, you get this:

 FAIL  src/useMyModule.test.js
  ● Test suite failed to run

    TypeError: (0 , _myModule.default) is not a function

      2 |
      3 | console.log(bar);
    > 4 | bar();
        |    ^
      5 |

      at Object.<anonymous> (src/useMyModule.js:4:4)
      at Object.<anonymous> (src/useMyModule.test.js:2:1)

So, how should I mock the foo function, but leave the default bar function intact?